This invention relates to phase-locking servo-hydraulic vibrator control systems, and more particularly to a phase detector for a vibrator control system.
In seismic geophysical surveying, utilizing a servo-control vibrator as a source of seismic waves, the need for maintaining a constant phase relationship between the reference or pilot signal and the output signal of the vibrator transmitted into the ground is well established. This phase-locking of the seismic waves to the reference signal is especially important at lower frequencies and when a plurality of vibrators are used simultaneously. In any case, the vibrator must be phase-locked to the reference signal if the desired acoustic wave is to be transmitted into the earth.
One form of vibrator controller is disclosed in U.S. Pat. No. 3,698,508, issued to R. A. Landrum, Jr., on Oct. 17, 1972. In this system, a signal derived from an accelerometer attached to a vibrator baseplate is phase-compared by means of an analog multiplier with a reference signal. The analog multiplier output is used to adjust the phase shift of the signal actually applied to the vibrator to thereby bring the vibrator baseplate into phase-lock with the reference signal. A first problem which occurs in all vibrator control systems is that the output of the accelerometer has an amplitude which varies with the driving frequency. This amplitude modulation passes through the multiplier and modulates the multiplier error signal output when it is desired that only phase changes cause error signal changes. The solution for this problem, as illustrated in the Landrum patent, is to pass the accelerometer output signal through an automatic gain control amplifier before coupling it to the phase detecting multiplier. The automatic gain control amplifier does improve the operation of this type of system, but, as recognized in the Landrum patent, the accelerometer output signal also contains substantial harmonic signals which, quite often, have amplitudes greater than the fundamental signal. These large harmonics interfere with the automatic gain control amplifier and result in suppression of the fundamental frequency signals to a level below that which is desired. In the Landrum patent, this harmonic interference problem is alleviated by introducing a tracking filter in the signal path from the accelerometer to the AGC amplifier. The tracking filter is a low pass filter with a variable cutoff point which is controlled in response to the reference signal to remove harmonics from the accelerometer output signal.
A second basic type of vibrator-control system is illustrated by the disclosure of U.S. Pat. No. 3,626,267, issued to J. T. Bobbitt, on Dec. 7, 1971. The Bobbitt patent illustrates a basically digital vibrator control system in which variable phase shifting is accomplished by changing the point at which a vibrator control signal is taken from a shift register. The phase detector of this control system is also digital and comprises basically a pair of zero crossing detectors and logic circuitry for generating pulses whose widths correspond to the time between zero crossings of the reference signal and the accelerator feedback signal. In the Bobbitt system, an automatic gain control amplifier is included to reduce the amplitude modulation normally present in the accelerator feedback signal. The system is still subject to interference from harmonics which can cause suppression of the fundamental signal in the automatic gain control amplifier. Of more importance in this type of phase detector is the fact that the harmonics shift the zero crossings of the feedback signal from the time at which they would occur if only the fundamental were present.
Thus, it is seen that in presently known control systems for vibrators the phase detectors are sensitive to amplitude modulation and harmonic distortion in the accelerometer feedback signal. Useful systems must include an automatic gain control amplifier and often a tracking filter to compensate for the amplitude modulation and harmonic content of typical accelerometer feedback signals.
Accordingly, an object of the present invention is to provide an improved phase detector for a servo-hydraulic vibrator.
Another object of the present invention is to provide a phase detector which detects the phase relation of a reference signal and the fundamental frequency component of a feedback signal which contains substantial harmonic components.